WASHINGTON — A first wave of additional U.S. troops will go into Iraq before the end of the month under President Bush's new war plan, a senior defense official said Tuesday. Congressional Democrats kept up their criticism of plans to add soldiers in the unpopular conflict.

Up to 20,000 troops will be put on alert and be prepared to deploy under the president's plan, but the increase in forces on the ground will be gradual, said the official, who requested anonymity because the plans have not yet been announced.

Details were emerging a day before Bush was to address the nation on his broad initiative to shore up the fragile country after nearly four years of bloodshed. Bush is expected to link the troop increase to moves by the Iraqi government to ease the country's murderous sectarian tensions, and to increased U.S. economic aid.

Moving first into Iraq would be the 2nd Brigade, 82nd Airborne Division, which is in Kuwait and poised to move quickly into the country, the defense official said.

Key congressional Democrats grappled Tuesday to frame a clear response to President Bush's expected plan to escalate U.S. military operations in Iraq with 20,000 more troops.

Sen. Edward M. Kennedy, D-Mass., led an effort to quickly challenge Bush's "new way forward" in the 46-month Iraq campaign by urging that the commander-in-chief be required to win congressional approval to send more troops or face loss of the federal funds to carry out the reinforcement.

Vermont's two U.S. senators and House member said Tuesday they oppose Bush's plan to add more troops in Iraq, and appeared poised to support efforts to block funding for that plan.

"The president is moving in exactly the wrong direction and it's appropriate for Congress to exercise its constitutional authority to rein him in," independent Sen. Bernie Sanders said in an interview.

Both he and Sen. Patrick Leahy, D-Vt., whose office issued a statement on the issue, said they would support Kennedy's proposal.

Rep. Peter Welch, D-Vt., said he, too, supported efforts to put the brakes on the White House. "I certainly would oppose funding for an escalation," he said in a phone interview.

Sen. Joseph Biden, D-Del., a prospective 2008 presidential candidate, called for Democrats to "generate overwhelming public support" for ending U.S. operations in Iraq.

"The thing that will force the president to change policy and disengage is when Republicans (stop backing his handling of the war,)" Biden said.

Senate Majority Leader Harry Reid, D-Nev., said a nonbinding resolution expected to reach the Senate floor next week would urge Bush not to send more troops.

"I really believe that if we can come up with a bipartisan approach to this escalation it will do more to change the direction in Iraq than anything else we can do," Reid said.

Kennedy's measure wouldn't block funding a troop increase outright. Instead, it would require new congressional approval for any such funding. But the measure's take on the war was clear in the "findings" contained in it.

When Congress last voted on the Iraq question in 2002, Kennedy's resolution said, "it authorized a war against the regime of Saddam Hussein because he was believed to have weapons of mass destruction and to have an operational relationship with Al-Qaeda."

The mission of the armed forces now "no longer bears any resemblance" to the mission then, it said.

Leahy and Sanders said they would need some Republican support for the Kennedy resolution. Chances were seen as slim that there would be enough GOP support to override a presidential veto.